The applied use of flywheels as a method of storing energy dates back to at least the use of potter's wheels. Modem flywheels are used as energy storage and power management devices. In essence, they couple a motor generator with a large-inertia rotor. Typical applications may include a vacuum enclosure or housing, friction-reducing elements such as magnetic bearings, coolant systems, sensors, charge/discharge circuitry, computer hardware and software for control. Flywheels systems can support industrial voltages.
Flywheels can be designed for primary application either as energy storage or as power storage units, the difference being in the relationship between the large-inertia rotor and the motor generator. Such designs take into consideration for example the time to charge up the device, the speed of the rotating mass, discharge time and the output.
One issue with flywheel systems is the potential of catastrophic structural failure of the large-inertia rotor. Upon failure of a rotor operating at high angular velocities, large forces are generated and large amounts of energy released which raise substantial issues of safety. These forces need to be managed and the energy safely dissipated to allow for broader application of such flywheel systems in industry.
The amount of energy stored in a flywheel is among other attributes proportional to the square of the angular velocity of the flywheel. Hence, flywheel systems are usually designed so that the flywheel rotates at or near a maximum practical velocity. For example, flywheels may be operated at angular velocities of 100,000 revolutions per minute or greater, and with rim velocities of 1,200 meters per second or greater.
The catastrophic structural failure of a flywheel rotating at high velocity (sometimes referred to as a “rapid disassembly” or “rotor burst”) will transform the flywheel into high-velocity debris comprising particles, fragments, and dust of various sizes and shapes (the particles, fragments, and dust formed in this manner are hereinafter referred to collectively as “debris particles,” for convenience).
The debris particles travel radially outward, i.e., away from the flywheel's axis of rotation, in a manner similar to a pressure wave. (The debris particles also include a tangential velocity component due to the rotation of the flywheel.) The aggregate kinetic energy of the debris particles is approximately equal to the kinetic energy of the flywheel at the time of its failure. Hence, a catastrophic structural failure of a rotating flywheel results in a substantially instantaneous release of the kinetic energy stored in the flywheel.
The high-velocity debris particles liberated from the flywheel can cause substantial damage and injury to nearby structures and personnel. Hence, many flywheel systems include some type of containment device that surrounds the flywheel. Containment devices are adapted to contain and de-energize at least a portion of the debris particles liberated from the flywheel during a catastrophic failure.
FIG. 1 depicts a particular type of conventional containment device 100. The containment device 100 is adapted for use in a test facility where flywheels may intentionally be operated to the point of failure. (This particular application is described for exemplary purposes only. Flywheel containment devices are also used in numerous other applications, e.g., in conjunction with flywheels operated as part of an uninterruptable power supply in an electrical substation or a factory.)
The containment device 100 comprises a substantially cylindrical outer housing 102, and a liner 104 positioned within and fixedly coupled to the outer housing 102. The liner 104 includes a first circumferentially-extending contoured portion 104b located proximate an upper edge of the liner 104. The liner 104 also includes a second circumferentially-extending contoured portion 104c located proximate a lower edge of the liner 104.
The containment device 100 further includes a lid 106 secured to an upper portion of the outer housing 102, and a base plate 107 secured to a lower portion of the outer housing 102. The base plate 107 is fixed to a mounting surface 125 by a plurality of mounting bolts 109.
A composite flywheel 108 is rotatably suspended from the lid 106, and is positioned within the volume defined by the liner 104, the lid 106, and the base plate 107. The flywheel 108 is adapted to rotate in the direction denoted by the arrow 120. (The various components that rotatably couple the flywheel 108 to the containment device 100 are not depicted in FIG. 1, for clarity.)
Composite materials are being used with increasing frequency in the manufacture of flywheels. Composite flywheels, as noted previously, typically disintegrate into a relatively large number of debris particles upon catastrophic structural failure. One such debris particle is depicted diagrammatically in FIG. 1, and is denoted by the numeral 110. (Flywheels formed of steel or other metallic materials, by contrast, usually undergo a so-called “tri-hub” failure in which the flywheel breaks into three relatively large shrapnellike pieces.) The rotational motion of the flywheel 108 at the time of failure gives the debris particles 110 a velocity vector having both radial and tangential components. Hence, the debris particles 110 travel radially outward, toward an inner surface 104a of the liner 104, immediately after the time of failure. (The path of travel of the debris particles 110 is denoted by the arrows 112 included in FIG. 1.)
Initial contact between the inner surface 104a of the liner 104 and the high-velocity debris particles 110 typically deflects the debris particles 110 in the axial (“y”) direction. Hence, most of the debris particles 110 initially travel upward and downward, toward the contoured portions 104b, 104c of the liner 104. The curvilinear profiles of the contoured portions 104b, 104c alters the course of the debris particles 110. More particularly, the contoured portions 104b, 104c turn and substantially reverse the course of the debris particles 110. Hence, the debris particles 110 are not stopped by the lid 106 or the base plate 107. The liner 104 thereby reduces or eliminates the stress and shock loading on the lid 106 and the base plate 107 that would otherwise be caused by contact with the high-velocity debris particles 110.
The contoured portions 104b, 104c redirect, but do not substantially dissipate the kinetic energy of the debris particles 110. Most of the kinetic energy of the debris particles 110 is dissipated by collisions between the debris particles 110 and the various other portions of the containment device 100 and/or other debris particles. The components of the containment device 100 must have a thick or otherwise robust construction to withstand these collisions, and to absorb the energy associated therewith. This type of construction makes the containment device 100 heavy, and can thereby cause difficulties in transporting or otherwise moving the containment device 100. Excessive weight can also necessitate reinforcement of the mounting surface 125 surface and the mounting bolts 109. Furthermore, increasing the thickness of a structural component requires additional material, and thereby raises the initial cost of the containment device 100.
Dissipating the kinetic energy of the debris particles 110 through collisions with the containment device 100 raises safety-related issues. In particular, the collisions between the high-energy debris particles 110 and the components of the containment device 100 generates a risk that one or more of the debris particles 110 may penetrate and thereby escape from the containment device 100. Upon escape, the high-velocity debris particles 110 from the containment device 100, as noted previously, can cause substantial property damage and injury to personnel.
Furthermore, as indicated the failure of a high-speed rotor and associated hub structure causes debris to initially depart from the flywheel 108 in a trajectory essentially tangential to the rotor. The debris mass then contacts the enclosure wall closest to the rotor, generally a few degrees less than perfectly tangential. The forces imparted to the wall of the housing impact a substantial tangential force, which tends to rotate the housing. With further reference to FIG. 1, the tangential velocity component of the debris particles 110 exerts a torque on the containment device 100. In extreme cases, this very high torsional stress on the housing mounting may subject it to failure by shear or separation of the containment device 100 from its mounts. If the mounting is designed to fully constrain this torsional load, it becomes excessively hefty and overweight for many practical flywheel applications.
This problem is of concern in the industry, hence an ongoing need exists for a flywheel system with a housing capable of absorbing and/or dissipating the energy inherent in the failure of high speed rotor, without causing substantial property damage and injury to personnel, and avoiding the problems of such heavily-engineered housings as to be impractical for use